Signal cutout device, optical connector and optical fiber coupler

ABSTRACT

A signal cut-off device  4  includes an optical fiber which has a clad  42  on the outer periphery of a core  41 , and first and second variation sections of a refractive index,  41   a  and  41   b  in which the period of a grating varies gradually along an optical axis are serially formed in the longitudinal direction of the core  41.    
     An optical connector which is detachably connected to the predetermined place of an optical transmission line includes a housing which has a plug-styled portion at a front end part and a jack-styled portion at a rear end part, and a ferrule which is mounted in the plug-styled portion. The signal cut-off device  4  is installed in the hole of the ferrule. An optical fiber coupler includes a coupler body which multiplexes/demultiplexes an uplink signal (1260-1360 nm) and a downlink signal (1480-1580 nm), a COM port which is provided on the input side of the coupler body, and a 1.55 port and a 1.3 port which are provided on the output side of the coupler body. A second connector is mounted on the end part of an optical fiber constituting the COM port, a PD is mounted on the end part of an optical fiber constituting the 1.55 port, and an LD is mounted on the end part of an optical fiber constituting the 1.3 port.

TECHNICAL FIELD

The present invention relates to a signal cut-off device, an opticalconnector and an optical fiber coupler, and it relates to a signalcut-off device, an optical connector and an optical fiber coupler whichcan reliably cut off an image, a signal or the like transmitted from theside of a station in, for example, an FTTH (Fiber To The Home) systemthat applies a B-PON (Broadband-Passive Optical Network) system.

BACKGROUND ART

In recent years, a WDM (wavelength division multiplexing) access systemcalled “B-PON” has been limelighted as a system which transmitslarge-capacity information.

FIG. 10 shows a schematic configurational diagram in the case where theB-PON system is applied to an optical subscriber access system of FTTB(Fiber To The Building). Referring to the figure, the system includes anoptical line terminal (hereinbelow, termed “OLT”) 1 which is disposed onthe side of a station, and a plurality of optical network units(hereinbelow, termed “ONUs”) 2 which are disposed on the sides ofsubscribers. The OLT 1 and the respective ONUs 2 are connected throughoptical transmission lines 3. By the way, in FIG. 10, only one ONU 2 isillustrated for the brevity of description.

The OLT 1 includes a PD (Photo Diode) for reception (hereinbelow, termed“station-side PD”) 11, an LD for transmission (hereinbelow, termed“station-side LD”) 12, a WDM coupler (hereinbelow, termed “station-sidecoupler”) 13 which multiplexes/demultiplexes two waves of uplink anddownlink, and a PLC (planar lightwave circuit) splitter 14, while theONU 2 includes a PD for reception (hereinbelow, termed “subscriber-sidePD”) 21, an LD for transmission (hereinbelow, termed “subscriber-sideLD”) 22, and a WDM coupler (hereinbelow, termed “subscriber-side WDMcoupler”) 23 which multiplexes/demultiplexes two waves of uplink anddownlink. Here, the station-side PD 11 and the station-side LD 12 areconnected to the station-side WDM coupler 13, to which the PLC splitter14 is connected. Besides, the subscriber-side PD 21 and thesubscriber-side LD-22 are connected to the subscriber-side WDM coupler23, to which the PLC splitter 14 is connected through the opticaltransmission line 3. By the way, in the figure, signs 24 a and 25 adenote first and second connectors attached to the distal ends ofoptical fiber cords, respectively, and sign 26 a denotes an adaptor forconnecting the first and second connectors 24 a and 25 a.

In the B-PON system of such a configuration, as shown in FIG. 11(a), aband of 1260-1360 nm is used for an uplink signal from the ONU 2 to theOLT 1, and a band of 1480-1580 nm is used for a downlink signal from theOLT 1 to the ONU 2.

Meanwhile, the B-PON system of such a configuration is currentlyimplemented for business users (FTTB) by way of trial, but it is underinvestigation to divert the FTTB system to general users (FTTH) forversion-up in the future.

Since, however, the delivery of an image signal is not considered insuch an FTTB system, the diversion to the FTTH system necessitates that,as shown in FIG. 11(b), a band (1550-1560 nm) for the delivery of animage signal is secured as the band of the existing signal is secured bycompressing the used band of the downlink signal to 1480-1500 nm.

On the other hand, in such an FTTH system, some of the users do notrequire the “delivery of an image signal”, so that the image signal of1550-1560 nm needs to be cut off for such users.

Here, for the cut-off of the image signal, an isolation of at least 40dB needs to be secured in the wide band of 1550-1560 nm, while the FTTHsystem is supposed to be mounted under the eaves or the like of thehouse of each user, so that also the installation environment of thesystem, especially the working temperatures thereof (−40° C.-85° C.)need to be considered.

A filter of multi-layer dielectric films and a fiber Bragg grating (FBG)can be mentioned for optical devices which meet such requisites.

Since, however, the former optical device is used with the filter ofmulti-layer dielectric films inserted between optical fibers and fixedby a binder or with this filter held between connectors, it might causea failure or its characteristics might change under the severetemperature environment.

Besides, in the latter optical device, the fiber Bragg grating which iselongate must be used for securing the isolation of at least 40 dB inthe wide band of 1550-1560 nm, so that in the manufacture of such afiber Bragg grating, a phase mask having the special specification of achirp rate of at most 6.3 nm/cm is required, resulting in the drawbacksof an inferior yield and an increase in cost.

By the way, when a grating is written with a phase mask whose chirp rateis 5.5 nm/mm, an isolation of 43 dB is anticipated in computation.However, when a grating was actually written with a phase mask whosechirp rate was 5.6 nm/mm, thereby to fabricate a fiber Bragg gratingwhose band was 1650±5 nm, the lowest isolation was about 30 dB.

Further, in such an FTTH system, it is necessary to suppose the user'schange that the “delivery of an image signal” not having been requiredbecomes required. In such a case, the splicing work of the fiber Bragggrating needs to be redone, to incur the drawback that the configurationof the FTTH system cannot be changed-over with ease.

Besides, in the FTTH system, digital transmission is performed, and itis hardly influenced by reflection (total reflection) ascribable to thecut-off wavelength of the fiber Bragg grating, but suppressing suchreflection to the utmost is ideal.

The present invention has been made in order to solve the drawbacksstated above, and it has for its object to provide a signal cut-offdevice, an optical connector and an optical fiber coupler which canreliably cut off the wavelength of an image signal by at least 40 dB ina wide band of 1550-1560 nm for users not requiring “delivery of animage signal” in, for example, an FTTH system that applies a B-PONsystem, and with which an initial system not requiring the “delivery ofan image signal” can be easily and inexpensively changed-over midway toa system requiring the “delivery of an image signal”.

DISCLOSURE OF THE INVENTION

In order to accomplish such an object, a signal cut-off device accordingto the present invention consists in that a first variation section of arefractive index, in which a period of a grating varies gradually alongan optical axis, is provided in a core of an optical fiber, and that asecond variation section of the refractive index, in which the period ofthe grating varies gradually along the optical axis, is provided inseries with the first index variation section.

Besides, each of the first and second index variation sections in thesignal cut-off device of the present invention is arrayed from along-wave-region side toward a short-wave-region side in an opticalwaveguide direction.

According to the signal cut-off device of the present invention, thefirst and second index variation sections in which the period of thegrating varies gradually along the optical axis are serially provided inthe core of the optical fiber, so that a signal, such as an image signalwhich is transmitted from an optical line terminal, can be reliably cutoff.

An optical connector according to the present invention consists incomprising a housing which includes a plug-styled portion at a front endpart and a jack-styled portion at a rear end part, and a ferrule whichis mounted in the plug-styled portion, wherein a signal cut-off deviceis installed in a hole of the ferrule.

According to the optical connector of the present invention, theplug-styled portion and the jack-styled portion are included, so thatthe optical connector can be detachably connected to an adaptor and aconnector which are disposed in the predetermined places of an opticaltransmission line. As a result, an FTTH system can be easily andinexpensively constructed by diverting an FTTB system which is currentlyin operation, and an FTTH system which does not require “delivery of animage signal” can be easily changed-over to an FTTH system whichrequires the “delivery of an image signal”.

An optical fiber coupler according to the present invention consists, inan optical fiber coupler having a port, in that a first variationsection of a refractive index, in which a period of a grating variesgradually along an optical axis, is provided in a core of an opticalfiber constituting the port, and that a second variation section of therefractive index, in which the period of the grating varies graduallyalong the optical axis, is provided in series with the first indexvariation section.

Besides, each of the first and second index variation sections in theoptical fiber coupler of the present invention is arrayed from along-wave-region side toward a short-wave-region side in an opticalwaveguide direction.

According to the optical fiber coupler of the present invention, thefirst and second index variation sections in which the period of thegrating varies gradually along the optical axis are serially provided inthe core of the optical fiber, so that a signal, such as an image signalwhich is transmitted from an optical line terminal, can be reliably cutoff. Besides, the port itself of the optical fiber coupler serves as anoptical waveguide, so that the coupler can be easily connected to anoptical fiber which is an optical transmission line. As a result, anFTTH system can be easily and inexpensively constructed by diverting anFTTB system which is currently in operation, and an FTTH system whichdoes not require “delivery of an image signal” can be easilychanged-over to an FTTH system which requires the “delivery of an imagesignal”.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a signal cut-off device accordingto the present invention.

FIG. 2 is an explanatory view of an optical connector according to thepresent invention, wherein FIG. 2(a) is a side view of the opticalconnector, and FIG. 2(b) is a front view of the optical connector.

FIG. 3 is an explanatory diagram showing the cut-off characteristics ofthe optical connector (signal cut-off device) according to the presentinvention.

FIG. 4 is a configurational diagram of a B-PON system (FTTH system notrequiring delivery of an image signal) according to the presentinvention.

FIG. 5 is a configurational diagram of a B-PON system (FTTH systemrequiring the delivery of an image signal) according to the presentinvention.

FIG. 6 is a model diagram of an optical fiber coupler according to thepresent invention.

FIG. 7 is an explanatory view showing the manufacturing steps of theoptical fiber coupler according to the present invention, wherein FIG.7(a) is an explanatory view showing a state where one of two opticalfibers is provided with first and second variation sections of arefractive index, FIG. 7(b) is an explanatory view showing a state wherean optical branching/coupling portion is formed by fusing together andstretching the two optical fibers, FIG. 7(c) is an explanatory viewshowing a state where part of the other optical fiber is cut away, FIG.7(d) is a side view showing a state where the optical branching/couplingportion is packaged, and FIG. 7(e) is a sectional view taken along lineA-A in FIG. 7(d).

FIG. 8 is an explanatory view showing the cut-off characteristics ofoptical fiber couplers, wherein FIG. 8(a) is an explanatory view showingthe cut-off characteristics of the optical fiber coupler in the priorart, and FIG. 8(b) is an explanatory view showing the cut-offcharacteristics of the optical fiber coupler according to the presentinvention.

FIG. 9 is a configurational diagram of a B-PON system (FTTH system notrequiring delivery of an image signal).

FIG. 10 is a configurational diagram of a B-PON system (FTTB system) inthe prior art.

FIG. 11 is a waveform diagram of uplink and downlink signals in a B-PONsystem.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, preferred embodiments of a signal cut-off device, an opticalconnector and an optical fiber coupler according to the presentinvention will be described with reference to the drawings.

FIG. 1 shows a vertical sectional view of a signal cut-off deviceaccording to the present invention. Referring to the figure, the signalcut-off device 4 according to the present invention includes an opticalfiber which has a clad 42 on the outer periphery of a core 41 thatcontains quartz-based glass as its main ingredient. In the core 41,first and second variation sections of a refractive index, 41 a and 41 bare formed serially in the longitudinal direction of this core 41, inthe following way: A standard wide-band mask (not shown) whose chirprate is 11 nm/cm is disposed outside the optical fiber, and the opticalfiber is irradiated with ultraviolet radiation from outside thewide-band mask, whereby the first index variation section 41 a in whichthe period of a grating varies gradually along an optical axis can beformed in the predetermined place of the core 41. Subsequently, theoptical fiber is shifted in its axial direction in this state, and it isirradiated with ultraviolet radiation similarly to the above, wherebythe second index variation section 41 b in which the period of thegrating varies gradually along the optical axis can be formed.Incidentally, three or more index variation sections may well be formedby a similar method.

Here, each of the first and second index variation sections 41 a and 41b is provided so as to gradually vary from a long-wavelength-side regionto a short-wavelength-side region in the proceeding direction A oflight, and a flat region (region where the grating is not written) 41 chaving a predetermined length (about 1 mm) is provided between the firstindex variation section 41 a and the second index variation section 41b.

In the signal cut-off device 4 of such a configuration, the fiber Bragggrating which can secure an isolation of at least 40 dB can bemanufactured by the standard wide-band phase mask whose chirp rate is 11nm/cm, so that a yield does not lower, and a manufacturing cost is notapprehended to increase. Moreover, since the grating is written in thecore of the optical fiber, the signal cut-off device can be easilymounted in a ferrule. Further, the characteristics of the signal cut-offdevice can be stabilized more than by the cut-off of an image signalbased on a filter of multi-layer dielectric films.

FIG. 2(a) shows a side view of an optical connector which employs thesignal cut-off device 4 thus manufactured, while FIG. 2(b) shows a frontview of the optical connector. Referring to the figures, the opticalconnector 6 according to the present invention includes a housing 7which has a plug-styled portion 71 at its front end part, and ajack-styled portion 72 at its rear end part. An elongate ferrule (about22.4 mm) constructed of a zirconia ferrule or the like is mounted in theplug-styled portion 71 of the housing 7.

The ferrule 8 is provided with a hole 81, in which the signal cut-offdevice (fiber grating) 4 stated before is inserted and fixed.

Here, the first and second index variation sections 41 a and 41 b of thesignal cut-off device 4 are mounted so as to be arrayed from thelong-wave-region side onto the short-wave region side in the proceedingdirection A of the light. In this embodiment, the first index variationsection 41 a is mounted facing the side of the plug-styled portion 71,and the second index variation section 41 b facing the side of thejack-styled portion 72. The influence of reflection ascribable to thegrating can be relieved about 2 dB by mounting the signal cut-off device4 within the housing 7 in such a state. This phenomenon is interpretedto occur on the basis of the fact that, when the signal light is enteredfrom the long-wave-region side, it will undergo a radiation mode loss inpassing through the grating, whereupon the resulting reflected lightwill be emitted from the clad 42 without being propagated in the core41.

The ferrule 8 should desirably be designed so as to flexibly conform toany connector such as FC connector or SC connector, and the signalcut-off device (fiber grating) 4 should desirably be put into the shapeof one package and mounted in the ferrule as a plug type.

According to the optical connector of such a configuration, as will bestated later, the signal cut-off device can be detachably connected to aconnector which is disposed in the predetermined place of an opticaltransmission line.

FIG. 3 shows the cut-off characteristics of the optical connector(signal cut-off device) of such a configuration.

It is understood from the figure that, according to the opticalconnector (signal cut-off device) of the present invention, the imagesignal can be cut off within a range of about 40-55 dB in a wide band of1550-1560 nm.

Accordingly, when the optical connector (signal cut-off device) of sucha configuration is used, the image signal which is transmitted from anoptical line terminal on a station side can be reliably cut off on asubscriber side.

FIGS. 4 and 5 show configurational diagrams of B-PON systems in the casewhere the optical connector of the present invention is applied to anFTTH system. By the way, in these figures, the same signs are assignedto portions which are common to those in FIG. 10, and which shall beomitted from detailed description.

Referring to FIG. 4, the B-PON system includes an OLT 1 disposed on theside of a station, and a plurality of ONUs 2 disposed on the sides ofsubscribers, and the OTL 1 and the individual ONUs 2 are connected byoptical transmission lines 3 each being made of a single-mode opticalfiber or the like. By the way, in FIG. 4 and in FIG. 5 to be referred tolater, only one ONU 2 is illustrated for the brevity of description.

The OLT 1 includes a first station-side PD 11 which receives an uplinksignal of 1.3 μm band, a first station-side LD 12 which transmits adownlink signal of 1.49 μm band, a station-side WDM coupler 13 whichmultiplexes/demultiplexes two waves of uplink and downlink, a PLCsplitter 14, and a second station-side LD 15 which transmits a downlinksignal of 1.55 μm band, while the ONU 2 includes a first subscriber-sidePD 21 which receives the downlink signal of 1.49 μm band, asubscriber-side LD 22 which transmits the uplink signal of 1.3 μm band,a subscriber-side WDM coupler 23 which multiplexes/demultiplexes the twowaves of uplink and downlink, and the optical connector 6 of the presentinvention.

Here, the station-side PD 11 and the first station-side LD 12 areconnected to the station-side WDM coupler 13, to which the PLC splitter14 and the second station-side LD 15 are connected through astation-side coupler (splitter) 16. Besides, the first subscriber-sidePD 21 and the subscriber-side LD 22 are connected to the subscriber-sideWDM coupler 23, to which the optical connector 6 of the presentinvention is connected through a second connector 25 a. Further, theoptical connector 6 is connected to a first adaptor 26 a, to which thePLC splitter 14 is connected through a first connector 24 a as well asthe optical transmission line 3.

In the B-PON system of such a configuration, the optical connector 6 isprovided with the plug-styled portion 71 (refer to FIG. 2) and thejack-styled portion 72 (refer to FIG. 2), so that the plug-styledportion 71 of the optical connector 6 can be connected to the firstadaptor 26 a disposed in the predetermined place of the opticaltransmission line 3, while the second connector 25 a can be detachablyconnected to the jack-styled portion 72 of the optical connector 6.

Consequently, according to the B-PON system of such a configuration, theFTTH system which does not require “delivery of an image signal” can beeasily and inexpensively constructed in such away that the opticalconnector 6 of the present invention is detachably connected to theadaptor and connector of the optical transmission line as constitute anFTTB system which is currently in operation.

Next, in a case where the FTTH system not requiring the “delivery of animage signal” is to be changed-over to an FTTH system requiring the“delivery of an image signal”, the optical connector 6 of the presentinvention may be detached from the FTTH system shown in FIG. 4,whereupon as shown in FIG. 5, the second connector 25 a is connected toa filter (or coupler) 28 through a second adaptor 26 b as well as athird connector 24 b, a second subscriber-side PD 27 which receives thedownlink signal of 1.55 μm band is connected to the filter (or coupler)28, and the filter (or coupler) 28 is connected to the first adaptor 26a through a fourth connector 25 b.

Thus, the FTTH system not requiring the “delivery of an image signal”can be easily and inexpensively changed-over to the FTTH systemrequiring the “delivery of an image signal”.

FIG. 6 shows a model diagram in the case where an optical fiber coupleraccording to the present invention is applied to an FTTH system. By theway, in the figure, the same signs are assigned to portions which arecommon to those in FIGS. 1, 4 and 5, and which shall be omitted fromdetailed description.

Referring to FIG. 6, the optical fiber coupler 5 of the presentinvention includes a coupler body 51 which multiplexes/demultiplexes anuplink signal (1260-1360 nm) and a downlink signal (1480-1580 nm), a COMport 52 which is located on the input side of the coupler body 51, andfirst and second OUT ports 53 and 54 which are located on the outputside of the coupler body. Here, a second connector 25 a is mounted onthe end part of an optical fiber constituting the COM port 52, while aPD 21 is mounted on the end part of an optical fiber constituting thefirst OUT port 53 (hereinbelow, termed “1.55 port”), and an LD 22 ismounted on the end part of an optical fiber constituting the second OUTport 54 (hereinbelow, termed “1.3 port”).

The optical fiber coupler 5 of such a configuration can be manufacturedin a way shown in FIG. 7. By the way, in the figure, the same signs areassigned to portions common to those in FIG. 1 and FIGS. 4-6.

First, as shown in FIG. 7(a), two optical fibers 4 a and 4 b in each ofwhich the periphery of a bare optical fiber of single mode is coatedwith a resin are prepared, and first and second variation sections of arefractive index, 41 a and 41 b are serially written into the core ofone 4 a of the optical fibers by a method to be stated later. Besides,the resin coatings of the intermediate parts of the two optical fibers 4a and 4 b are removed over predetermined lengths, thereby to denude thebare optical fibers 4 a′ and 4 b′.

Subsequently, the two bare optical fibers 4 a′ and 4 b′ are fusedtogether and stretched while being melted by a micro burner equipment orthe like, and the stretching is stopped at the position of apredetermined branching ratio. Thus, as shown in FIG. 7(b), an opticalbranching/coupling portion B, and first-fourth optical fiber portions 6a-6 d extending from both the sides of this portion are obtained. Here,as shown in FIG. 7(c), that part of the other optical fiber 4 b′ whichis joined to the second optical fiber portion 6 b is cut away.

Subsequently, as shown in FIGS. 7(d) and (e), the opticalbranching/coupling portion B and the first-fourth optical fiber portions6 a-6 d are accommodated in a groove 61 a which is provided in a packagebase 61 made of pure quartz or the like, and the first-fourth opticalfiber portions 6 a-6 d are fixed to the package base 61 through binderportions 62 a and 62 b, whereupon the resulting structure isaccommodated in a stainless steel tube 63 or the like. Incidentally, aprotective tube 64 such as shrinkable tube is disposed on the outerperiphery of the stainless steel tube 63 as may be needed. Thus, it ispossible to obtain the optical fiber coupler 5 which has the COM port52, the 1.55 port 53 and the 1.3 port 54.

Next, the method of writing the grating into the core of the opticalfiber constituting the COM port 52 will be described.

First, the optical fiber 4 is formed in such a structure that, as in thesignal cut-off device shown in FIG. 1, a clad 42 is provided on theouter periphery of a core 41 which contains quartz-based glass as itsmain ingredient, and the outer periphery of the clad 42 is coated withan UV-hardenable resin or the like resin (not shown). Besides, the resincoating at a part where the grating is to be written is peeled off todenude the clad 42, and a standard wide-band mask (not shown) whosechirp rate is 11 nm/cm is disposed outside the clad 42. When the opticalfiber is irradiated with ultraviolet radiation from outside thewide-band mask in this state, the first variation section of arefractive index, 41 a in which the period of the grating variesgradually along an optical axis is formed in the predetermined place ofthe core 41.

Subsequently, the optical fiber 4 is shifted in its axial direction inthis state, and it is irradiated with ultraviolet radiation similarly tothe above, whereby the second variation section of the refractive index,41 b in which the period of the grating varies gradually along theoptical axis is formed in series with the first section 41 a.Incidentally, after the first and second index variation sections 41 aand 41 b have been formed, the optical fiber 4 is subjected to recoating55 (refer to FIG. 6) with the UV-hardenable resin or the like, and ifnecessary, the resulting structure is subjected to packaging with ametal or the like.

Here, as in the foregoing, each of the first and second index variationsections 41 a and 41 b is provided so as to gradually vary from along-wavelength-side region to a short-wavelength-side region in theproceeding direction A of light, and a flat region (region where thegrating is not written) 41 c having a predetermined length (about 1 mm)is provided between the first index variation section 41 a and thesecond index variation section 41 b.

In the optical fiber coupler having such two index variation sections,the first and second index variation sections 41 a and 41 b are seriallyformed in the core of one optical fiber, and hence, the fiber Bragggrating which can secure an isolation of at least 40 dB can bemanufactured by the standard wide-band phase mask whose chirp rate is 11nm/cm, with the result that a yield does not lower, and a manufacturingcost is not apprehended to increase. Moreover, since the grating iswritten in the core of the optical fiber, the characteristics of theoptical fiber coupler can be stabilized more than by the cut-off of animage signal based on a filter of multi-layer dielectric films. Further,in the case where each of the first and second index variation sections41 a and 41 b is provided so as to gradually vary from thelong-wavelength-side region to the short-wavelength-side region in theproceeding direction A of the light, the influence of reflectionascribable to the grating can be relieved about 2 dB. This phenomenon isinterpreted to occur on the basis of the fact that, when the signallight is entered from the long-wave-region side, it will undergo aradiation mode loss in passing through the grating, whereupon theresulting reflected light will be emitted from the clad 42 without beingpropagated in the core 41.

FIG. 8(a) shows the cut-off characteristics of an optical fiber couplerin the prior art, while FIG. 8(b) shows the cut-off characteristics ofthe optical fiber coupler according to the present invention. By theway, in each of FIGS. 8(a) and (b), a fine line indicates the cut-offcharacteristics of the 1.3 port, and a thick line indicates the cut-offcharacteristics of the 1.55 port.

First, regarding the prior-art optical fiber coupler, it is understoodfrom FIG. 8(a) that the 1.3 port transmits the signal of 1.3 nm bandnearly 100%, whereas it transmits no signal in the 1.55 nm band, andthat the 1.55 port transmits the signal of 1.55 nm band nearly 100%,whereas it transmits almost no signal in the 1.3 nm band.

Next, regarding the optical fiber coupler of the present invention, itis understood from FIG. 8(b) that the 1.3 port exhibits the same cut-offcharacteristics as those of the prior-art optical fiber coupler, butthat the 1.55 port hardly transmits the signal of 1.3 nm band, while itextracts the signal in the 1.55 nm band.

FIG. 9 shows a configurational diagram of a B-PON system in the casewhere the optical fiber coupler of the present invention is applied toan FTTH system. By the way, in FIG. 9, the same signs are assigned toportions which are common to those in FIGS. 4 and 6, and which shall beomitted from detailed description.

Referring to FIG. 9, the B-PON system includes an OLT 1 disposed on theside of a station, and a plurality of ONUs 2 disposed on the sides ofsubscribers, and the OTL 1 and the individual ONUs 2 are connected byoptical transmission lines 3 each being made of a single-mode opticalfiber or the like. By the way, in FIG. 9, only one ONU 2 is illustratedfor the brevity of description.

TheOLT 1 includes a first station-side PD 11 which receives an uplinksignal of 1.3 μm band, a first station-side LD 12 which transmits adownlink signal of 1.49 μm band, a station-side WDM coupler 13 whichmultiplexes/demultiplexes two waves of uplink and downlink, a PLCsplitter 14, and a second station-side LD 15 which transmits a downlinksignal of 1.55 μm band, while the ONU 2 includes a first subscriber-sidePD 21 which receives the downlink signal of 1.49 μm band, asubscriber-side LD 22 which transmits the uplink signal of 1.3 μm band,and the subscriber-side WDM coupler 5 of the present invention.

Here, the station-side PD 11 and the first station-side LD 12 areconnected to the station-side WDM coupler 13, to which the PLC splitter14 and the second station-side LD 15 are connected through astation-side coupler (splitter) 16. Besides, the first subscriber-sidePD 21 and the subscriber-side LD 22 are connected to the subscriber-sideWDM coupler 5, to which a first adaptor 26 a is connected through asecond connector 25 a. Further, the first adaptor 26 a is connected to afirst connector 24 a, to which the PLC splitter 14 is connected throughthe optical transmission line 3.

In the B-PON system of such a configuration, the fiber Bragg grating andthe coupler are made unitary, in other words, the first and second indexvariation sections 41 a and 41 b are serially formed in the core of theoptical fiber constituting the COM port of the subscriber-side WDMcoupler 5, so that the downlink signal of 1.55 nm band can be cut off.Besides, the connector is connected to the end part of the COM port, sothat it can be detachably connected to the connector disposed on theside of the optical network unit of the optical transmission line 3.

Consequently, according to the B-PON system of such a configuration, theFTTH system which does not require “delivery of an image signal” can beeasily and inexpensively constructed in such a way that the opticalfiber coupler 5 of the present invention is detachably connected to theadaptor and connector of the optical transmission line as constitute anFTTB system which is currently in operation.

Next, in a case where the FTTH system not requiring the “delivery of animage signal” is to be changed-over to an FTTH system requiring the“delivery of an image signal”, the optical fiber coupler 5 of thepresent invention may be replaced with the optical fiber coupler 23 inthe prior art as shown in FIG. 5 referred to before, whereupon thesecond connector 25 a is connected to a filter (or coupler) 28 through asecond adaptor 26 b as well as a third connector 24 b, a secondsubscriber-side PD 27 which receives the downlink signal of 1.55 μm bandis connected to the filter (or coupler) 28, and the filter (or coupler)28 is connected to the first adaptor 26 a through a fourth connector 25b.

Thus, the FTTH system not requiring the “delivery of an image signal”can be easily and inexpensively changed-over to the FTTH systemrequiring the “delivery of an image signal”.

Incidentally, although the foregoing embodiments have been described asto the cases of applying the signal cut-off device and the optical fibercoupler to the B-PON system, the present invention is not restricted tothe application, but it may well be applied to another system which cutsoff any signal other than the image signal. Besides, the opticalconnector 6 is not restricted to the connection to the opticaltransmission line 3 between the subscriber-side WDM coupler 23 and thePLC splitter 14, but it may well be connected to the opticaltransmission line between the subscriber-side WDM coupler 23 and thesubscriber-side PD 21. Further, the first and second index variationsections 41 a and 41 b are not restricted to the formation on the COMport side, but they may well be formed on the 1.55 port side. Inaddition, the number of the index variation sections to be formed is notrestricted to two, but three or more index variation sections may wellbe formed as may be needed.

Industrial Applicability

As understood from the above description, according to the signalcut-off device of the present invention, first and second variationsections of a refractive index, in which the period of a grating variesgradually along an optical axis, are serially provided in the core of anoptical fiber, so that a signal, such as an image signal which istransmitted from an optical line terminal, can be reliably cut off.Besides, according to the optical connector of the present invention, aplug-styled portion and a jack-styled portion are included, so that theoptical connector can be detachably connected to an adaptor and aconnector disposed in the predetermined places of an opticaltransmission line. As a result, an FTTH system can be easily andinexpensively constructed by diverting an FTTB system currently inoperation, and an FTTH system not requiring “delivery of an imagesignal” can be easily changed-over to an FTTH system requiring the“delivery of an image signal”. Further, according to the optical fibercoupler of the present invention, first and second variation sections ofa refractive index, in which the period of a grating varies graduallyalong an optical axis, are serially provided in the core of an opticalfiber, so that a signal, such as an image signal which is transmittedfrom an optical line terminal, can be reliably cut off. Besides, theport itself of the optical fiber coupler serves as an optical waveguide,so that the optical fiber coupler can be easily connected to, forexample, an optical transmission line constituting a B-PON system. As aresult, an FTTH system can be easily and inexpensively constructed bydiverting an FTTB system currently in operation, and an FTTH system notrequiring “delivery of an image signal” can be easily changed-over to anFTTH system requiring the “delivery of an image signal”.

1. A signal cut-off device characterized in that a first variationsection of a refractive index, in which a period of a grating variesgradually along an optical axis, is provided in a core of an opticalfiber, and that a second variation section of the refractive index, inwhich the period of the grating varies gradually along the optical axis,is provided in series with the first index variation section.
 2. Asignal cut-off device as defined in claim 1, characterized in that eachof the first and second index variation sections is arrayed from along-wave-region side toward a short-wave-region side in an opticalwaveguide direction.
 3. An optical connector characterized by comprisinga housing which includes a plug-styled portion at a front end part and ajack-styled portion at a rear end part, and a ferrule which is mountedin said plug-styled portion, wherein the signal cut-off device asdefined in claim 1 is installed in a hole of said ferrule.
 4. In anoptical fiber coupler having a port, an optical fiber couplercharacterized in that a first variation section of a refractive index,in which a period of a grating varies gradually along an optical axis,is provided in a core of an optical fiber constituting the port, andthat a second variation section of the refractive index, in which theperiod of the grating varies gradually along the optical axis, isprovided in series with the first index variation section.
 5. An opticalfiber coupler as defined in claim 4, characterized in that each of thefirst and second index variation sections is arrayed from along-wave-region side toward a short-wave-region side in an opticalwaveguide direction.